It is very likely that you've never had paella — at least not paella in the sense that the good people of the southeastern Spanish seaside capital of Valencia (pictured below) and the surrounding countryside, who invented it in the first place, would define it.
Like so many other emblematic traditional dishes — bouillabaisse, salade niçoise, fettuccine Alfredo, chili con carne — paella is a dish so frequently adapted, misinterpreted, gussied-up, and bastardized that the glorious original that got all the attention in the first place has become all but forgotten in most quarters. I've had paella made with long-grain and even instant rice; I've had it made with chicken stock, which would be blasphemy to a Valencian; I've seen recipes for Italian and Asian-fusion paella; the Los Angeles Times even once ran one for a quick-and-easy version made with leftover turkey, canned chopped clams, pepperoni, and Spanish rice mix — cooked in a microwave, of course. (Photo: Flickr/C.Y.R.I.L.)
Paella valenciana — the real thing — is one of the great rice dishes of the world, as honest and straightforward, and as definitive of the culture that created it, as a classic risotto milanese or a Korean bibimbop. Even in Spain itself, it is often misunderstood, badly cooked, and/or needlessly overladen with ingredients. The paella I'm talking about is not a baking dish full of yellow rice loaded with shrimp, clams, mussels, sausage, pork loin, chicken, peas, red peppers, and a few other things. It includes no seafood at all, in fact. In its purest, most authentic form, it is made with chicken, but the other usual ingredients are just rabbit, three kinds of beans (white beans, like the Italian cannellini; broad green beans; and garrofons, which are like butter beans), and sometimes snails, enhanced with a bit of onion and tomato and frequently but not always flavored with saffron. Aficionados will tell you that it's okay to sprinkle in some fresh rosemary, too, but only if you're not using snails — because the snails that would end up in paella are plucked from the fields, where rosemary would have been part of their diet, so they'd be bringing their own.
The rice used for paella, of course, must be a Valencian variety. Bomba is the best-known kind, but I know paella cooks who think it absorbs liquid too quickly and who prefer other local cultivars like fonsa or gleva. And note that there are two kinds of bomba, that from Valencia and that from Calasparra, in Murcia, to the south. A Valencian would no sooner use Murcian rice than a Bostonian would put tomatoes in his clam chowder. The cooking medium for the rice is plain water. Valencians will tell you, in fact, that only water from their aquifiers has precisely the right mineral content to produce perfect paella.
Some other rules for paella valenciana, at least in Valencia: It must be cooked over a hot wood fire, often of dried vine cuttings. (The smoke actually faintly flavors the rice.) Even though it uses short-grain rice like risotto does, and though risotto must be stirred constantly, paella is never stirred once the ingredients are assembled. It is never eaten hot, but allowed to sit for at least 30 or 40 minutes and often let cool to room temperature. And, at least in its home region, it is eaten only for lunch, never dinner, the theory being that it's entirely too rich a dish for an evening meal.
Paella-lovers will argue endlessly about where to find the best example of the dish, but my vote would go to a little place called Restaurante Levante, in Benisanó, 15 miles or so northwest of Valencia. Here, you will find paella valenciana made according to all the rules and strictures mentioned above, by Rafael Vidal, who was once named Mejor Paellero, or best paella maker, for the entire Valencia region, and who has cooked paella for the King of Spain.